Lecture #04:

Loops

2

C# Control Structures: Repetition

T

F

while structure

T

F

do/while structureF

T

for structure/foreach structure

3

while Statement

The while statement has the following syntax:

while ( condition ) statement;

whilewhile is a is areserved wordreserved word

If the condition is true, the statement is executed.If the condition is true, the statement is executed.Then the condition is evaluated again.Then the condition is evaluated again.

The statement (or a block of statements) is executed The statement (or a block of statements) is executed repetitively until the condition becomes false. repetitively until the condition becomes false.

4

while Statement (cont’d)

true

false

Product = 2 * productProduct <= 1000

int product;

product = 2;

while (product <= 1000)

{

product = 2 * product;

}

// beginning of the next statement

5

while Statement

Note that if the condition of a while statement is false initially, the statement is never executed

Therefore, the body of a while loop will execute zero or more times

6

Infinite Loops

The body of a while loop must eventually make the condition false

If not, it is an infinite loop, which will execute until the user interrupts the program

This is a common type of logical error You should always double check to ensure that

your loops will terminate normally

7

Example 1: Counter Controlled While Loop

Control variable• The variable used as a counter to determine

whether or not the loop should continue

Three components• Initial value of the counter• Check whether or not the counter has reached

target– When the looping should continue

• Incrementing/decrementing of the counter

8

Example 2: Sentinel Controlled while Loops

This is typical of an input-driven program

Continues an arbitrary amount of times

Sentinel value• Causes loop to break• Avoid collisions

– When flag value = user entered value

9

The do Statement

The do statement has the following syntax:

do{ statement;}while ( condition );

Uses bothUses boththe the dodo and andwhilewhile

reservedreservedwordswords

The statement is executed once initially, then the condition is evaluatedThe statement is executed once initially, then the condition is evaluated

The statement is repetitively executed until the condition becomes falseThe statement is repetitively executed until the condition becomes false

10

do/while Flowchart

true

false

action(s)

condition

Fig. 5.13 Flowcharting the do/while repetition structure.

11

Comparing the while and do Loops

The while loops vs. the do/while loops Using a while loop

• Condition is tested• The action is performed• Loop could be skipped altogether

Using a do/while loop• Action is performed• Then the loop condition is tested• Loop will be run at least once

T

F

while structure

T

F

do/while structure

Question: write a program to get max from user and then print the numbers from 1 to max

12

The for Statement

The for statement has the following syntax:

for ( initialization ; condition ; increment ) statement;

ReservedReservedwordword

The The initializationinitialization portion portionis executed onceis executed once

before the loop beginsbefore the loop begins

The statement isThe statement isexecuted until theexecuted until the

conditioncondition becomes false becomes false

The The incrementincrement portion is executed at the end of each iteration portion is executed at the end of each iteration

13

Flowchart of a for loop

increment

condition action(s)true

false

initialization

for ( initialization ; condition ; increment ) action(s);

14

The for Statement: Example

counter++

Establish initial value of control variable.

Determine if final value of control variable has been reached.

counter <= 10

Console.WriteLine( counter * 10 );

true

false

int counter = 1

Body of loop (this may be multiple statements)

Increment the control variable.

for (int counter = 1; counter <= 10; counter++)

Console.WriteLine (counter * 10);

// beginning of the next statement

15

The for Statement

A for loop is equivalent to the following while loop:

initialization;while ( condition ){ statement; increment;}

16

The for Statement

It is well suited for executing a specific number of times that can be determined in advance Increment/Decrement

• When incrementing– In most cases < or <= is used

• When decrementing– In most cases > or >= is used

17

The flexibility of the for Statement Each expression in the header of a for loop is

optional If the initialization is left out, no initialization is

performed If the condition is left out, it is always considered to be

true, and therefore creates an infinite loop If the increment is left out, no increment operation is

performed

Both semi-colons are always required in the for loop header

for ( ; ; ){

// do something

}

18

A Problem to Think About

How to print this?

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

What about this?

xxxxxxxxxxxxxxx xxxxxxxxxxxxx

xxxxxxxxxxx xxxxxxxxx xxxxxxx xxxxx xxx x

19

Statements break and continue

Used to alter the flow of control The break statement

• Used to exit a loop early

The continue statement• Used to skip the rest of the statements in a loop

and restart at the first statement in the loop

Programs can be completed without their usage; use with caution.